Blow molding is a well-known technique for forming articles from thermoplastic resins. For instance, it is known that double-walled containers may be formed by a blow molding process using a series of clamping and holding steps during extrusion of a parison, such as the blow-molded container is shown and described in U.S. Pat. No. 4,828,786 to Legg et al., the disclosure of which is incorporated herein by reference. One such blow-molded double-walled container may be used as a lockable fire-resistant safe for storing documents or other valuables which may be damaged or destroyed by exposure to high temperatures. For example, various types of fire-resistant safes are available from John D. Brush & Co., Inc, d/b/a/Sentry Group, Rochester, N.Y. 14625, USA.
A typical fire-resistant safe includes top and bottom elements that are hingedly coupled to one another to define a storage cavity therebetween. Each of the top and bottom elements are typically blow-molded to form a hollow space formed between inner and outer shells. The hollow space of each element between the shells is filled with a non-combustible, fire-proof thermally-insulating material such as hydrated Portland cement. Some examples of existing fire-resistant safes are disclosed in U.S. Pat. No. 4,805,290 to Brush, Jr. et al., U.S. Pat. No. 5,295,447 to Robbins et al., and U.S. Pat. No. 6,752,092 to Beattie et al., the disclosures of which are hereby incorporated by reference.
It may be desirable in some instances to securely attach an object to either of the top and bottom elements of the fire-resistant safe (e.g., a bungee cord). One way to attach an article to a blow-molded container is to use one or more separate attaching mechanisms that are fastened to the container either before or after the hollow space is filled with fire-resistant insulation. Specifically, a screw or other fastener would be drilled through either the inner or outer shell to secure the attaching mechanism to the container. The object may then be attached to the container using the attaching mechanisms.
This particular solution is problematic in that it requires one or more separate attaching mechanisms to be mounted to the container, which results in an increase in production cost. Moreover, using separate attaching mechanisms with the container lengthens the manufacturing time of the container due to the additional time required to mount the attaching mechanism and the clean-up time that is involved due to the drilling that is required to secure the attaching mechanism to the container.
Another way to attach an article to a blow-molded container is to blow-mold the attaching mechanism, such as an undercut, into the inner or outer shell of the container. Although blow-molding provides a useful method of manufacturing shaped plastic articles, problems are frequently encountered when the article to be formed includes undercuts that extend from the outer surface of the article. In particular, when undercuts are molded into the inner or outer shell of a container using the blow-molding process, the undercut itself tends to lock the article in the mold, thereby hindering or even preventing its removal from the mold.
A variety of approaches have been taken to deal with the problems caused by forming an undercut in a blow-molded container. For example, U.S. Pat. No. 4,101,256 to White et al. describes the use of a blow mold having hinged portions to facilitate removal of a molded product having an undercut feature from the mold. In another example, U.S. Pat. No. 7,101,168 to Willemsen describes the use of a mold that includes a plurality of segments which are movable in a radial direction so that the molded article can be removed from the mold. However, each of the approaches set forth above involve molds utilize a certain amount of mechanization, which increases the manufacturing costs and complicates the molding process.
Accordingly, there exists a need for an undercut feature for a blow-molded double walled container that is formed in the container during the blow molding process, and that is capable of being removed from the blow mold without mechanization. The present invention fills these needs as well as other needs.